1. Field of the Invention
The present invention is concerned with the field of power electronics. It relates to a converter with a DC voltage intermediate circuit, comprising a rectifier circuit having an AC voltage input and a DC voltage output, which rectifier circuit can be connected to a power supply system, an invertor circuit having a DC voltage input and an AC voltage output, and a DC voltage intermediate circuit which is arranged between the DC voltage output of the rectifier circuit and the DC voltage input of the invertor circuit and comprises two connecting lines leading from the DC voltage output of the rectifier circuit to the DC voltage input of the invertor circuit, and at least one intermediate circuit capacitor arranged between the connecting lines.
The invention furthermore relates to a method for operating such a converter.
2. Discussion of Background
Converters with a DC voltage intermediate circuit have been known and used for a long time in power electronics. In said converters, the AC voltage from a power supply system is converted into a DC voltage in a rectifier circuit on the input side and smoothed by means of an intermediate circuit capacitor in the subsequent DC voltage intermediate circuit. The DC voltage across the intermediate circuit capacitor is then converted into another AC voltage in a subsequent invertor circuit.
In the event of a short circuit in the invertor, nowadays use is made for example of fuses on the DC voltage side of the rectifier circuit. Another option is to use fuses in the AC leads from the power supply system to the rectifier circuit. A so-called fuseless circuit is also possible. In this case, the short circuit has to be disconnected on the power supply system side by a power circuit-breaker, but in this case the reactance of the power feeding must be large enough to prevent an impermissibly high short-circuit current. Before the closing of the power circuit-breaker on the power supply system side, the intermediate circuit capacitor is charged by an auxiliary power supply unit to a voltage which is large enough to prevent impermissible overcharging after connection of the power circuit-breaker. It is also known from low-voltage converters to replace at least three diodes of the rectifier circuit by thyristors, at least one of which is bridged by a high-value resistor. After connection of the power circuit-breaker, the intermediate circuit capacitor is charged via said resistor. Afterwards the thyristors receive a continuous trigger pulse and are operated as diodes.
In the case of the known solutions, however, various problems arise with regard to the protection function and the charging function:
Fuses for interrupting medium-voltage circuits are expensive. Moreover, they have to be replaced in the event of a fault or short circuit, which increases the costs for the stocking of spare parts and repairs. PA0 Fuses on the AC side isolate only the converter from the power-feeding system. During a short circuit in the invertor, the majority of the energy of the intermediate circuit capacitor is transferred to the inductances contained in the shorted circuit. After the zero crossing of the capacitor voltage, the current commutates into the diodes of the rectifier circuit. It remains there until all of the energy is converted into heat. If the diodes of the rectifier circuit are only designed for normal operation, they can be described in this fault situation. The same phenomenon also occurs with a fuseless circuit. In both cases, this problem can be avoided by an inductor between the rectifier circuit and the intermediate circuit capacitor. However, an additional inductor considerably increases the costs of the converter. PA0 The additionally required auxiliary charging device increases the costs of the converter. PA0 Thyristors in the rectifier circuit likewise increase the equipment costs. PA0 Both the auxiliary charging device and the thyristors in the rectifier circuit have no significance for all the other (normal) operating states.